In recent years, organic materials and nutrients such as nitrates, nitrites, ammonia and phosphates which are contained in industrial effluent and domestic waste water in large amounts are pointed out as a cause of eutrophication leading to red tide in the estuaries and a cause of pollution of lakes by stimulating a multiplication of algae. It is said that the critical concentration of nitrogen compounds (nitrates and nitrites) and phosphates are 0.15 ppm and 0.02 ppm, respectively. The establishment of technology for removing such organic materials and nutrients from the region of higher concentration to the region of lower concentration are earnestly desired.
Methods for removing phosphates from waste water are divided roughly into biological treatment methods and physicochemical treatment methods. A coagulating sedimentation method using coagulant to remove phosphates as slightly soluble salts is a general method as physicochemical treatment methods from the viewpoint of economy and efficiency. These methods have been utilized in processes such as effusion of salts from coagulants to waste water, sludge treatment, recovery and recycle of phosphates, and insufficient removal of phosphates in the region of lower concentration thereof.
A method for adsorbing phosphates using a phosphate adsorbent to remove phosphates may also be accomplished. In carrying cut the adsorption method, aluminum hydroxide gel, magnesium oxide, titanium oxide-active carbon complex, zirconium oxide-active carbon complex, volcanic ash soil and the like are examined as a phosphate adsorbent.
In recent years, a particularly noted water-related environmental problem has been the eutrophication in the closed water areas such as swamp and lakes, rivers and estuaries. The Japanese government has enacted laws to address this problem. Restrictions concerning the discharge of phosphates, have become more strict. The present process for treating waste water without dephosphorilation is not enough to meet the new restriction with respect to phosphate. Therefore, an appropriate phosphate-treating step is needed. Under such background, a phosphate adsorbent capable of efficiently recovering and removing phosphates from waste water and capable of recycling phosphates from the viewpoint of effective use of resources is needed. In addition, a process for post-treatment of the phosphate adsorbent is needed. The inflow of phosphate-containing waste water from areas where there is sewage but no equipment for treating waste water or where there are limitations due to land form and scattered houses are major causes of eutrophication. Since treatment of waste water relies on a septic tank at each house in the area, an incorporation of a dephosphorilation step in the phosphate-treatment procedure at a septic tank is important. In that case, a high-performance and renewable phosphate adsorbent is needed.
1.0 to 1.5 m.sup.3 drain having a phosphate concentration of 5-500 ppm per month per general house is discharged in a farm area. With respect to a large-sized septic tank applied to a group of 10 people or more, phosphates may be removed by coagulation and sedimentation using coagulant containing iron salt or aluminum salt as a major ingredient. In contrast, with respect to a small-sized septic tank applied to a group of 5-10 people, a coagulant may not be substantially used to remove phosphates because of low efficiency, requiring a phosphate adsorbent. Conditions needed for the phosphate adsorbent applied to a small-sized septic tank are basically the same as a phosphate adsorbent applied to an equipment for waste water treatment. Since the septic tank may not be enlarged and frequency and cost of maintenance are important, the phosphate adsorbent should not require enlargement of the small-sized septic tank or increase frequency and cost of maintenance. Further, removal of phosphates nitrates and nitrites is preferred, since they also cause eutrophication. As the pressure for lower phosphate concentration increases it is important that methods be developed to decrease phosphate concentration to not more than 0.02 ppm, which is the critical concentration for the generation of red tide. Therefore, phoshate adsorbents which are useful for not only removal of phosphate from waste water with a higher phosphate concentration, but also from rivers with a lower phosphate concentration, need to be developed.
It is an object of the invention to provide a reasonable method for treating phosphate-containing waste water by a phosphate adsorbent having a large capacity of phosphates adsorption and recycling the phosphate adsorbent.
It is another object of the invention to provide an agent for treating waste water which is capable of removing phosphates, nitrates and nitrites .